Lipo discharge /storage

[fly boy3]

hi all, my basic budget charger has no facility to discharge lipos down to storage volts. What is the best and safest way to go about it. Is it feasible to install lipo In a model, run the motor and check the volts periodically ? Or is there a better way. Cheers

Edited By fly boy3 on 16/03/2016 16:54:43

[Jack Banner]

Just buy a new charger. Most of the basic 4 button jobs have a discharge facility and can be bought quite cheaply.

[Andy48]

... or simply don't bother discharging. You probably do more harm continually charging/discharging than doing nothing.

[MattyB]

Get a few of these dischargers from MyRCMart or eBay - simple, effective, safe and saves wear on your charger. I have three, they are excellent.

Edited By MattyB on 16/03/2016 19:22:55

[MattyB]

Posted by Andy48 on 16/03/2016 18:49:28:

... or simply don't bother discharging. You probably do more harm continually charging/discharging than doing nothing.

Not correct, especially if they are high c rated cells and are left for long periods. If you are leaving your batteries for longer than 48-72hrs between charging you are going to increase the cell IR and decrease capacity, current delivery capability and pack life in a relatively short period of time. See the battery university site for lots of good info on this.

[fly boy3]

Thanks all for replies on this post. I think Jack is correct , I need a new charger with more functions. LoL. It is surprising that most of the electric fliers I know have said they all agree with the principle of lipo storage, but not one carries it out. I wonder why ? Cheers

[Andy48]

Posted by MattyB on 16/03/2016 19:27:06:

Posted by Andy48 on 16/03/2016 18:49:28:

... or simply don't bother discharging. You probably do more harm continually charging/discharging than doing nothing.

Not correct, especially if they are high c rated cells and are left for long periods. If you are leaving your batteries for longer than 48-72hrs between charging you are going to increase the cell IR and decrease capacity, current delivery capability and pack life in a relatively short period of time. See the battery university site for lots of good info on this.

There's a huge difference between "long periods" and 48-72hours in my book.

One thing that is certain, repeated charging and discharging will reduce the life of the battery. The number of charging cycles for lipos is relatively small.

My point is, how much benefit is there in prolonging battery life by discharging to a storage level, and over what period of time is this benefit seen, against the obvious wear and tear you impose on the battery by simply discharging and recharging for storage.

[fly boy3]

I see your point Andy. I am an ic nut and have very few lipos, but I still want to learn about them. Perhaps the normal use of the lipo during the flying season, and storage volts for Winter lay off should be the way to go ? Cheers

Edited By fly boy3 on 19/03/2016 12:18:03

[Andy48]

That sounds more reasonable, that's what I do.

The other issue is that discharging a full battery takes ages as the discharge rate is low, and as you are supposed not to leave the battery unattended, that brings in its own set of problems.

If you've got a DC 12v source, one of these from Hobbyking is brilliant. They show a graph of the charge/discharge rate as its charging and the colour screen is a touch screen. There's £10 off at the moment from Hong Kong, though they are sold out in the UK. The only problem is that it doesn't come with balance leads.

http://www.hobbyking.co.uk/hobbyking/store/__58187__X120_120W_Touch_Screen_Smart_6S_Balance_Charger.html

 

Edited By Andy48 on 19/03/2016 12:39:33

[Piers Bowlan]

Everyone seems to be talking about discharging to a storage voltage. As has already been pointed out discharging a fully charged LiPo can take some time. Perhaps a silly question but do some chargers have the function to charge to storage voltage once a pack has been discharged after a flying session. Next question, what is the 'correct' storage voltage?

[Piers Bowlan]

I think I can answer that now... 3.85v per cell and I think I am going to have to buy a new charger with a 'charge to storage voltage' charger!

[Dane Crosby]

Hi, I have a reasonably priced charger which will charge a LiPo to the storage level (3.8v/cell). It will discharge a full battery and charge a used battery to this level on the <Storage> setting. It is an EQ mini and I've had it for 18 months with no snags. I tend to charge/discharge to storage if I know that I'm not going to use the batteries within 28 days.

I work from the free download information sheets from 4Max (Purple Power). These are in PDF format and are quite comprehensive. I have followed the suggestions therein and my LiPos are in very good health. I do log each use, time used, end voltage per cell, and charge amount (I know, a little bit OCD) and they have all been used about the same amount of time and have been charged on average 35 times so far with no perceptible drop in performance. I have two 3S3300, three 3S2200, two 3S1800 and a 2S 2100 and they are all OK after between 12 and18 months in service.

I hope that this is of some help.

[MattyB]

Posted by Andy48 on 19/03/2016 09:27:29:

One thing that is certain, repeated charging and discharging will reduce the life of the battery. The number of charging cycles for lipos is relatively small.

My point is, how much benefit is there in prolonging battery life by discharging to a storage level, and over what period of time is this benefit seen, against the obvious wear and tear you impose on the battery by simply discharging and recharging for storage.

Short answer - there is plenty of benefit for the average w/e flier to discharging to storage, provided you treat your packs correctly in all other respects too. Examples...

When my Dad got his first electrics he used to charge straight after flying to be ready for the next session. As a result all of those packs puffed and/or had dramatically reduced capacity and current delivery capability within ~100-150 cycles at around the 1 year mark. By contrast I have 4 Gens Ace 3S 2600 25C packs bought when I first started flying electric around 4 years ago. They have always been discharged to storage voltage if returning from the field unused, are discharged at an average of ~10-15C in the model. I set my timer so that they come out of the plane at no lower than 3.8V/cell resting, avoiding the need to do any further charging or discharging at home to put them in storage state.

3 of these packs now have 300-400 flights on them, and have between 85-90% of rated capacity - I would say they will easily get to 600 cycles on them. Modern batteries are even better - some have tested the new Turnigy Graphene packs on the bench, and after 1000+ high C rated discharges they are still showing ~85% capacity and very low increase in cell IR! Interestingly one of my Gens Ace packs has ~300 flights but higher measured IR and noticeably softer performance in the same model in the air, and guess what - that is the pack I over discharged to ~3.0V/cell due to an error not resetting the timer on one flight!

To me this shows that there clearly is a value in storing at 3.8-3.85V/cell, but only if you are careful to look after your batteries in all other ways too I.e don't over discharge or try and pull currents from them beyond ~50% of the rated (which as we know are mostly made up!) C rating. For small packs you may choose to abuse them a bit as they are cheap enough not to worry, but if moving beyond 3S it's clear to me that there is significant benefit in caring for you packs carefully. YMMV.

[fly boy3]

Thanks Dane, you have just justified that I need a new charger for this season. 4MattyB, very interesting post. I must say. Just to sum up what you said (I,m old ) lol, if you fly your model, you will use yout timer so at the ennd of the flight the lipo is near to the discharge volts, and if you do not fly you will discharge lipo at home? Cheers

[Andy48]

Posted by MattyB on 20/03/2016 10:36:19:

Posted by Andy48 on 19/03/2016 09:27:29:

One thing that is certain, repeated charging and discharging will reduce the life of the battery. The number of charging cycles for lipos is relatively small.

My point is, how much benefit is there in prolonging battery life by discharging to a storage level, and over what period of time is this benefit seen, against the obvious wear and tear you impose on the battery by simply discharging and recharging for storage.

Short answer - there is plenty of benefit for the average w/e flier to discharging to storage, provided you treat your packs correctly in all other respects too. Examples...

When my Dad got his first electrics he used to charge straight after flying to be ready for the next session. As a result all of those packs puffed and/or had dramatically reduced capacity and current delivery capability within ~100-150 cycles at around the 1 year mark. By contrast I have 4 Gens Ace 3S 2600 25C packs bought when I first started flying electric around 4 years ago. They have always been discharged to storage voltage if returning from the field unused, are discharged at an average of ~10-15C in the model. I set my timer so that they come out of the plane at no lower than 3.8V/cell resting, avoiding the need to do any further charging or discharging at home to put them in storage state.

3 of these packs now have 300-400 flights on them, and have between 85-90% of rated capacity - I would say they will easily get to 600 cycles on them. Modern batteries are even better - some have tested the new Turnigy Graphene packs on the bench, and after 1000+ high C rated discharges they are still showing ~85% capacity and very low increase in cell IR! Interestingly one of my Gens Ace packs has ~300 flights but higher measured IR and noticeably softer performance in the same model in the air, and guess what - that is the pack I over discharged to ~3.0V/cell due to an error not resetting the timer on one flight!

To me this shows that there clearly is a value in storing at 3.8-3.85V/cell, but only if you are careful to look after your batteries in all other ways too I.e don't over discharge or try and pull currents from them beyond ~50% of the rated (which as we know are mostly made up!) C rating. For small packs you may choose to abuse them a bit as they are cheap enough not to worry, but if moving beyond 3S it's clear to me that there is significant benefit in caring for you packs carefully. YMMV.

All this is purely anecdotal, and with so many unstated variables, of very little value.

[MattyB]

Posted by fly boy3 on 20/03/2016 11:44:07:

...MattyB, very interesting post. I must say. Just to sum up what you said (I,m old ) lol, if you fly your model, you will use your timer so at the end of the flight the lipo is near to the discharge volts, and if you do not fly you will discharge lipo at home?

Yes, basically. A few of my bigger aircraft have cell telemetry so I know exactly the cell voltage in flight, but I still have a timer set so that I land with between 3.8-3.85V/cell remaining. Obviously though it's important to reduce your timer by 15-20% in very cold conditions though, otherwise you could still overdischarge your pack. Any packs I have left unflown at the end of a session I always discharge unless I am flying again within 48hrs, in which case I just leave em be.

[Simon Chaddock]

I would also endorse the concept of treating a battery as if its C rating was only half that claimed.

There are batteries with a claimed 65C constant discharge. That is a full discharge in just 80 seconds. It might be able to do that - but I wonder how many times?

[MattyB]

Posted by Andy48 on 20/03/2016 14:32:04:

All this is purely anecdotal, and with so many unstated variables, of very little value.

I am not sure why the burden of proof is higher for me than for you - where is your evidence that cycling is more damaging than long term high voltage storage? Anyway, here is a good page from Battery University (whose writers have probably forgotten more about batteries than you or I will ever know) on the subject - a quote from halfway down...

"Lithium-ion suffers from stress when exposed to heat, so does keeping a cell at a high charge voltage. A battery dwelling above 30°C (86°F) is considered elevated temperature and for most Li-ion a voltage above 4.10V/cell is deemed as high voltage. Exposing the battery to high temperature and dwelling in a full state-of-charge for an extended time can be more stressful than cycling. Table 3 demonstrates capacity loss as a function of temperature and SoC."

Edited By MattyB on 20/03/2016 19:35:22

[Andy48]

Posted by MattyB on 20/03/2016 19:31:57:

I am not sure why the burden of proof is higher for me than for you - where is your evidence that cycling is more damaging than long term high voltage storage?

Exposing the battery to high temperature and dwelling in a full state-of-charge for an extended time can be more stressful than cycling.

I never claimed to have proof, I was simply pointing out that the act of charging/discharging is a major factor in the life cycle of a battery. I don't know the answer and would like to see some REAL evidence either way. Even in your above quote, "extended time" is not defined. I very strongly suspect this is measured in weeks and months rather than hours or days.

[PatMc]

Posted by Simon Chaddock on 20/03/2016 19:27:48:

I would also endorse the concept of treating a battery as if its C rating was only half that claimed.

There are batteries with a claimed 65C constant discharge. That is a full discharge in just 80 seconds. It might be able to do that - but I wonder how many times?

If the battery is used for an E-glider in one of the limited power run comps it may only be used at high discharge rate for between 4 or 5 up to 30 secs during a single flight. Such a high C rating means low ESR & the ability to use a small capacity (therefore lightweight) battery.

[MattyB]

Posted by Andy48 on 20/03/2016 20:06:42:

I never claimed to have proof, I was simply pointing out that the act of charging/discharging is a major factor in the life cycle of a battery. I don't know the answer and would like to see some REAL evidence either way. Even in your above quote, "extended time" is not defined. I very strongly suspect this is measured in weeks and months rather than hours or days.

You are almost certainly correct. But if I fly at w/es from March through to September as so many do and not discharge unflown packs between sessions, then those packs WILL spend multiple months per year above the critical 4.1V/cell at which performance affecting oxidation occurs at the cathode.

Another interesting use case of degradation due to the high voltage effect, this time from an electric vehicle, the Nissan Leaf.

 

Edited By MattyB on 20/03/2016 20:24:44

[fly boy3]

Big thanks to Matty B and other forum members for putting me straight re care of Lipos. As an ic man I do not have many, but did need to look after them correctly. Cheers

[Andy48]

I checked my batteries today before going down to the field. These 6 4S batteries are my main batteries that go in most of my planes, so have had a considerable use. Two have been in crashes and have extra shrink wrap round them, none are the slightest puffed up. They are now at least 4 years old, and cheap and cheerful, not expensive batteries. All were fine, IR is still low, about the same as a brand new Turnigy "A" spec still, using the Turnigy Lipo IR checker. I always charge them up fully, and I don;t think they have ever been storage charged. Using the on-board telemetry, they are still giving the expected current, and an 8 minute flight only used half of their 3000ah capacity. Looks like they'll do another season no problem.

In winter 2014 I bought a load of 3000 4S batteries that HK were selling off cheap, £12 each I think. I charged them up last spring expecting my current set to give up the ghost, and instead they have sat for a year unused in my heated workshop. I checked three of these over too, by first charging them. One took 38mah, one 6mah and the third showed full charge as soon as I connected it up. All had IR within a couple of mohm of each other, and all below that of the Turnigy "A" spec.

My conclusions from this anecdotal evidence:

1. Don't believe all the guff you read about such things, after all there is subtle but significant chemistry difference between each type and make, and they are constantly evolving.

2. Don't pay a fortune for "good" batteries, the cheap ones appear just as good.

3. Use a good charger that charges to 4.1v, not 4.2v or higher, and don't believe what it says in the blurb or on screen. My Turnigy/ Imax (see link above) charger charges to 4.12v.

4. Misuse in flight is the quickest way to ruin batteries. I always fit a smaller prop which "detunes" the motor somewhat so that it never draws max current, and well below the battery C rating. The loss of top speed is very minor, but the extended flight time and battery life is significant.

5. My £100 worth of batteries have already worked out far cheaper than the equivalent in glow fuel.

6. Flying is all about enjoyment. I'd rather spend time building and flying, rather than faffing about with batteries.